Polypropylene is widely used in a variety of molding fields because of its superior mechanical properties, chemical resistance and cost-effectiveness. However, its melt strength is low, so that it is of inferior formability in thermoforming such as vacuum forming and pressure forming, blow molding and foam molding.
Some methods have been proposed to enhance the melt strength of polypropylene. For example, Japanese Laid-Open Patent Publication (Tokkai-Sho) Nos. 59-93711 and 61-152754 have disclosed a method of reacting polypropylene with an organic peroxide and a crosslinking assistant in a molten state. Japanese Laid-Open Patent Publication (Tokkai-Hei) No.2-298536 has disclosed a method of reacting semicrystalline polypropylene with a peroxide having a low degradation temperature in the absence of oxygen so as to produce polypropylene having free-end long branches and containing no gel.
Other methods for enhancing melting viscoelastic properties such as melt strength are, for example, a method of using a composition comprising polyethylenes or polypropylenes having different intrinsic viscosities or molecular weights, or a method of producing such compositions by multistage polymerization.
For example, Japanese Patent Publication (Tokko-Sho) No. 61-28694 has disclosed a method where 2 to 30 parts by weight of ultra high molecular weight polypropylene are added to 100 parts by weight of regular polypropylene and extrusion is performed in a temperature range from a melting point to 210.degree. C. Japanese Patent Publication (Tokko-Hei) No. 1-12770 has disclosed a method using multistage polymerization to obtain an extrusion sheet formed of two components of polypropylene having different molecular weights and an intrinsic viscosity ratio of at least 2. Japanese Patent Publication (Tokko-Sho) No. 62-61057 has disclosed a method of producing a polyethylene composition formed of three types of polyethylene having different viscosity average molecular weights comprising 1 to 10 wt % of high viscosity average molecular weight polyethylene by melting and kneading or multistage polymerization. Japanese Patent Publication (Tokko-Hei) No. 5-79683 has disclosed a method for polymerizing polyethylene in which an amount between 0.05 and 1 wt % of ultra high molecular weight polyethylene having an intrinsic viscosity of 20 dl/g or more is polymerized by multistage polymerization. Japanese Patent Publication (Tokko-Hei) No.7-8890 has disclosed a method for polymerizing polyethylene in which 0.1 to 5 wt % of ultra high molecular weight polyethylene having an intrinsic viscosity of 15 dl/g or more is polymerized by multistage polymerization in a polymerization reactor having a special arrangement by using a highly active titanium catalyst component prepolymerized with 1-butene or 4-methyl-1-pentene.
Furthermore, Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 5-222122 has disclosed a method for producing polypropylene having a high melt strength by polymerizing propylene by using a prepolymerized catalyst that results from the prepolymerization of ethylene and a polyene compound with a support type solid catalyst component comprising titanium and an organic aluminum compound catalyst component. Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 4-55410 has disclosed a method for producing ethylene .alpha.-olefin copolymer having high melt strength by using an ethylene-containing preliminary polymerization catalyst comprising a polyethylene with an intrinsic viscosity of at least 20 dl/g, wherein preliminary polymerization is performed exclusively in the ethylene by using the same catalyst as above.
An olefin (co)polymer having a good thermoformability for a sheet has been developed. For example, Japanese Patent Publication (Tokko-Sho) No. 56-15744 has disclosed a method of adding low density polyethylene and hydrated magnesium silicate powder to polypropylene. Japanese Patent Publication (Tokko-Sho) No. 63-29704 has disclosed a method of adding high density polyethylene and ethylene-propylene copolymer to polypropylene.
Although it has to be acknowledged that in the various components proposed in the prior art as described above and in the production processes belonging thereto the melt strength of polyolefin has been somewhat increased, many aspects such as a residual odor caused by the crosslinking assistant and forming-processability have io be improved.
Moreover, in a multistep polymerization that incorporates a production process of high molecular weight polyolefin into a main polymerization of a regular polypropylene (co)polymerization process, the precise control of the amount of olefin for (co)polymerization for formation of a small amount of high molecular weight polyolefin is difficult, and a low polymerization temperature is necessary for formation of the polyolefin with a sufficiently large molecular weight, which lowers the rate of production for the polypropylene composition. Thus, improvement of the process is necessary.
In the method for prepolymerizing a polyene compound, it is necessary to prepare the polyene compound separately. When propylene is polymerized according to a disclosed method of prepolymerizing polyethylene, the dispersibility of the prepolymerized polyethylene in a finally obtained polypropylene composition is not uniform, so that from the viewpoint of stability of the polypropylene composition, improvement of the process is required.
As has been pointed out above, the problem of low melt strength of polypropylene, as well as the problems of odor and low forming-processability are inherent in the prior art.